The present invention relates to a hybrid vehicle provided with an electric motor used to drive the vehicle and an internal combustion engine for power generation. More particularly, the present invention is directed to an operating method for an internal combustion engine for power generation of a hybrid vehicle, which permits an increased cruising range and improved power performances of the vehicle without abandoning an advantage of an electric vehicle an excellent exhaust gas characteristics.
Recently, the regulations on exhaust gas from those vehicles which use an internal combustion engine as their drive source have been made more rigorous to tackle environmental pollution. In this respect, various new technologies have been developed. Electric vehicles, which use an electric motor as their drive source and produce no exhaust gas, should be ideal motor vehicles for reduction of the exhaust gas amount. Typical electric vehicles are designed so that a battery is used to supply the electric motor with electric power. Naturally, however, the available battery capacity for each vehicle is limited, so that the power performances are poorer and the cruising range is shorter than those of the vehicles which use an internal combustion engine as the drive source. In order to make the electric vehicles more popular, these technical problems are expected to be solved.
Hybrid vehicles, which are furnished with a generator, driven by an internal combustion engine, for battery charging, are believed to be promising modern electric vehicles which can enjoy an increased cruising range.
Generally, hybrid vehicles are equipped with an exhaust gas purifier for improving the emission characteristics of the engine. If, however, the engine operation is started for the battery charging before the catalyst of the exhaust gas purifier is activated, the emission characteristics of the engine are spoiled. Even in the case where the engine operation is started after the catalyst is activated and properly functions, the purifying effect of the catalyst is lowered due to a deviation of the air-fuel ratio of a mixture supplied to the engine from a stoichiometric ratio, if the engine is in a cool state in which fuel evaporation is low while the amount of fuel supply is corrected to a larger value. In such cases, the emission characteristics are lowered.
In general, hybrid cars are designed so that the internal combustion engine for power generation is operated at a constant speed in order to improve its exhaust gas characteristics and reduce its fuel consumption. During such a constant-speed operation of the engine, the amount of electric power generated by the generator driven by the engine is substantially constant without regard to the operating state of the vehicle. Meanwhile, the vehicle is operated in various states. Thus, in a high-load operating state in which the electric motor for vehicle drive requires an increased electric power, the power generated by the generator may be smaller than the required electric power, making it necessary to supply electric power from the battery. Accordingly, if high-load operation is continued and the storage amount of the battery decreases, the amount of power supplied from the battery to the electric motor also decreases, thus lowering the power performances of the vehicle and shortening the cruising range. If the vehicle is stopped with the battery insufficiently charged, even the power supply to the catalyst heater for catalyst activation may possibly fails at restart of the vehicle. In such cases, the exhaust gas purifier does not function properly, so that exhaust gas containing pollutant or noxious gases is discharged from the engine when the engine is operated for the power generation.
In conventional hybrid vehicles, moreover, the operation of the power-generation engine is stopped when the driver turns off the starter key of the vehicle to stop the vehicle. Accordingly, when the starter key is turned off and the engine operation stops, the battery charging is forcibly terminated even though the engine had been operated for charging the battery at that time. Thus, the battery remains in an insufficiently charged state, possibly making it impossible to restart the vehicle.